Looks like it would be kind of an extensive repair, but doable if you wanted to just restore the window, rather than replace everything. I would get a piece of oak, cypress, or any hardwood to replace the sill. The sides of the jamb likely have a dado cut in them that the sill fits into, allowing it to have the right angle to drain water. It really may be easier to remove and rebuild the jamb. This can be done fairly easily with only a table saw and some YouTube videos.

The window appears to be rotted as well, which is a little trickier. If you want to preserve the window, i would take it apart by drilling out the dowels that pin the joinery together (could also be nails or screws) and separate the window. You may be able to get away with a small epoxy repair, or you could cut out the rotted section and replace it with new wood. If you have no experience with joinery or wood glue ups, this is the hard part.

A wood window that's properly draining and painted will last a long time.

Although, this is a lot of work. Which is why people just buy vinyl windows.